With the development of solid-state image sensing devices, image input equipment such as small video cameras for household use, digital cameras for personal computers, etc., have rapidly become widespread. In the case of the video camera, sufficient image quality can be obtained from a standpoint of television standard such as NTSC. CCD is normally used as the image sensing device. However, the CCD reads two fields twice as one frame corresponding to the NTSC and PAL methods, and the majority thereof has an oblong pixel.
However, recently, the object is to display images picked-up using a digital still camera on a display of a personal computer or the like. As a result, a CCD that reads all the square pixels (i.e. the progressive scan type) have been used as the image sensing device.
Thus, now a days the images picked-up with the digital still camera are mostly edited using a personal computer and printed using a printer. However, if a large-sized image is to be obtained, or a resolution that is necessary as a hard copy or computer graphics or the like is to be obtained, then the number of pixels provided in the present day digital still camera are not sufficient. A solid-state image sensing device having a higher number of pixels becomes necessary in these purposes. However, a screen size of the solid-state image sensing device becomes larger if the resolution is increased. As the screen size becomes larger, the lens section has to be made larger so that a down-sizing of the digital still camera becomes difficult. Moreover, a cost of such the solid-state image sensing device increases and can it not be used in the low-cost widespread digital cameras.
As a method of realizing high quality by the solid-state image sensing device, a so-called method of “pixel shift photography” has been well known. The pixel shift photography is a method of obtaining a high resolution by combining an image photographed by shifting a subject by half of a pixel pitch and an image before such shifting to obtain one image. Typical examples of the conventional image shift mechanism are shown in FIG. 27A to FIG. 27C. FIG. 27A shows a CCD shift method. In this CCD shift method, the image sensing device is shifted to obtain images that are different from each other. FIG. 27B shows a LPF gradient method. In this LPF gradient method, a LPF is inclined to obtain images that are different from each other. FIG. 27C shows a lens shift method. In this lens shift method, a lens is shifted to obtain images that are different from each other.
A desired image quality may not be obtained in the pixel shift photography because of the events such as a movement of hands, a movement of the subject, or a change in a performance of the pixel shift mechanism. In each of these events, the degree of deterioration of the image differs depending on the degree thereof. If the deterioration of the image crosses a certain level, the image may become inferior as compared to an image obtained in ordinary photography. The ordinary photography is the one that does not apply the technique of the pixel shift photography.
The cause of the change in the performance of the pixel shift mechanism may as follows. That is, an amount of shift of the pixel shift mechanism may change slightly due to a lapse of time, or the amount of shift of the pixel shift mechanism may change temporarily due to a change in the atmospheric temperature, or the pixel shift mechanism may fail due to vibration or impact and whereby the normal operation cannot be performed. The pixel shift mechanism requires micro-level displacement at high accuracy in comparison to any other mechanical sections of the digital camera. Therefore, the pixel shift mechanism is likely to be affected readily by the lapse of time, change in the temperature, and the influence of vibrations and impact. Thus, it is very likely that the pixel shift mechanism becomes defective permanently or temporarily.
For example, there is a case where a chemical reaction of curing progresses slightly even in a cured state of an adhesive, which would be judged as “being sufficiently cured” for normal use, and this slight difference will largely affect the pixel shift. Moreover, a slight change in rigidity of a structure due to temperature, a slight change in a coefficient of friction due to a change with the lapse of time, and a minute shift of a contact point of a structure due to an impact may cause a serious problem in the pixel shift, though these may be considered only as an error in a normal mechanism.
Moreover, if each of the movement of hands, the movement of the subject, or the change in the performance of the pixel shift mechanism is slight, although a higher quality can be obtained, the image quality is not near to the desired one. Deterioration of the quality of the images herein means a deterioration of resolution, an increase of blurred color, or the like.
FIG. 28A to FIG. 28C are diagrams for explaining the principle of pixel shift and image deterioration. A CCD having the Bayor array is taken as an example. As shown in FIG. 28A, in normal photographing, R, G and B signals of the CCD are interpolated to obtain R, G and B signals for each pixel. FIG. 28B shows an ideal state when the pixel shift photography (a shift by one pixel in the longitudinal direction) is performed. In the case of this pixel shift photography, an image having a high resolution can be obtained, and interpolation is not required. On the other hand, FIG. 28C shows a state when an accuracy of the pixel shift photography (a shift by one pixel in the longitudinal direction) is insufficient. In the case shown in FIG. 28C, the resolution of the image is deteriorated, and blurred color occurs.
A further problem is that a type of a LCD monitor normally mounted on a digital camera has a small number of displayed pixels. Therefore, even if there is a deterioration in the image quality, it can not be detected. In most cases, the deterioration in the image quality is recognized only after the image is transferred to a personal computer. Thus, failure of the pixel shift photography cannot be judged only with a digital camera (i.e., for example, the personal computer is required).